A field survey of changes in grain size of dry riverbed gravels was conducted with university students in the Seno River. The field survey results showed that gravels diameter tended to be smaller and rounder to downstream, and most students concluded that this reason was due to ‘crushing and abrasion’, based on what they had learnt in primary school, where large gravels were crushed and made smaller and rounder as they rolled and moved downstream. In order for gravels to undergo ‘crushing and abrasion’, they must first be transported across the riverbed. This requires ‘selective transport’, in which smaller particles are selectively transpoted. In order to understand how river work and to grasp the situation, it is important to consider many aspects of the river through observation.